Dynamic and depolarized dynamic light scattering colloid analyzer

ABSTRACT

Apparatus are described for measuring the characteristics of colloidal particles suspended in transparent media by Dynamic Light Scattering (DLS) and Depolarized Dynamic Light Scattering (DDLS) into regions where conventional measurements are difficult or impractical. Matching the diameter of an illuminating beam and an intersecting diameter of a field stop image extends measurements into regions that include concentrated turbid suspensions that frequently appear so visually opaque that multiple scattering typically gives a falsely low estimate of particle size. At the opposite extreme, where insufficient signal is available to determine either or both of the translational and/or rotational relaxation times of the particles, typically where they are too small, too few, or of insufficient refractive index difference from the medium to scatter enough light, measurements can be improved by: a) using a sufficiently large aperture such that many coherence areas fall upon the detector; and b) optical homodyne amplification of the scattered signal.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/861,079, filed Aug. 23, 2010, entitled “Dynamic and DepolarizedDynamic Light Scattering Colloid Analyzer,” by Smart et al., which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to the field of optical measurement andcharacterization of a variety of types of particles suspended in afluid. In particular, methods and apparatus are described for extendingconventional boundaries of particle hydrodynamic radius measurement toboth higher and lower particle concentration, and further for extendingdepolarized dynamic light scattering capabilities used for shapeassessment.

Dynamic Light Scattering (DLS) is used extensively in researchlaboratories and elsewhere for the development of new materials andprocesses, and less commonly is used for process monitoring and control.Commercial applications exist in several industries, including but notlimited to: pharmaceuticals (small molecules and protein therapeutics),medical diagnostics (histology, bodily fluids, cataracts), consumerproducts (personal care, cosmetics, paints, detergents), chemicals,environmental monitoring and remediation (particulate and biologicalpollutants, oil spill cleanup), advanced materials (powders, coatings,surfactants), and microelectronics (planarization slurries, thin films).Depolarized Dynamic Light Scattering (DDLS) is similar to DLS, but usespolarization techniques to assess deviations from particle sphericity.

DLS relies on the detection of the Doppler shift of coherent radiationscattered from small colloidal particles suspended in a transparentliquid and undergoing Brownian motion, whose behavior depends upon theirhydrodynamic radii and/or shape. DLS is commonly used for determiningthe translational diffusion coefficient of macromolecules such asproteins and polymers, as well as that of larger colloidal particles,typically up to several microns. Because the hydrodynamic radius of aspherical particle may be determined simply from its diffusioncoefficient (and the viscosity of the suspending liquid), dynamic lightscattering has become the method of choice for characterizing colloidalparticles. The phase and frequency of light scattered from manyparticles is detected as a fluctuating intensity in the far-field as thesuspended particles diffuse. DDLS acquires information about therotational diffusion coefficient of the particles, which depends on theparticle size and shape, and may be extracted by suitable mathematicaltechniques from the fluctuating intensity of the depolarized detectedlight.

DLS is a preferred measurement technique for the thermally drivendiffusion coefficient of particles in suspensions appearing translucent,those with an extinction length from a few millimeters to a few meters.The extinction length, based on Beer's Law, is that distance from theentrance into a medium to where the propagating beam intensity hasdeclined to e⁻¹ of its incident intensity. However, several importantapplication areas are outside these traditional limits of DLS. Forexample, there is great interest in measurement of suspensionsapproaching opacity, as in process monitoring of high concentrationslurries. As the concentration of particles in a suspension increases,the opportunity for scattered light to scatter from more than oneparticle before arriving at a detector also increases. The resultantmultiple scattering statistically yields a higher frequency signal and,consequently, a falsely low measurement of radius. At the oppositeextreme, there is also strong interest in the measurement of highlytransparent suspensions, such as for environmental monitoring and forcharacterization of dilute suspensions of nanoscale particles andproteins that scatter very little light. To date, technical solutions tomeasure accurately in both these regimes remain unsatisfactory.

DDLS has not yet become a popular method for measurement becausedepolarized signals are typically weak and therefore often obscured byinterfering signals such as stray light, optical imperfections, or othersystem noise sources. In addition, depolarized time correlationfunctions often decay many times more rapidly than those of DLS,especially for small particles. Increased frequency and reduced signalboth present challenges for typical detectors and their followingelectronics. Typical photodetectors optimized for weak signals, e.g.,photomultiplier tubes, also suffer from dead time, after-pulsing andnoise problems that arise from detecting a small number of photons percorrelation time of the relaxation process.

BRIEF SUMMARY OF THE INVENTION

The present invention introduces techniques and alignment proceduresapplicable to an apparatus for the extension of Dynamic Light Scattering(DLS) and Depolarized Dynamic Light Scattering (DDLS) for characterizing(e.g., determining particle size, size distribution, and/or particleaspect ratio) colloidal suspensions of particles into concentrationregions where conventional measurements are currently difficult orimpractical. The first region includes concentrated turbid suspensionsthat frequently appear visually opaque, and where excessive multiplescattering typically may give a falsely low estimate of particle size.The second region is the opposite extreme where insufficient signal isavailable to determine the rotational or even the translationalrelaxation time of the particles, typically when particles are toosmall, too few, have insufficient refractive index difference from thesuspending liquid, or where only the depolarized component is ofinterest. Separately or in combination, aspects of this inventionimprove both accuracy and the application range of DLS, and also extendDDLS to formerly inaccessible regions. The invention and its subsidiaryaspects described below enable an effective apparatus to be implementedmore simply than many others, allowing such possible configurations as aremote probe for process control in hostile environments, accuratecharacterization of extremely low volumes of specimen, measurement ofspecimens presented as a droplet on a transparent flat surface, orcontained within circular vials or other shaped cuvettes or capillaries,with improvements of accuracy of measurement of particle hydrodynamicradius, polydispersity, and deviations from sphericity, together withprediction of expected error boundaries, and other desirable advantages.While many DLS and DDLS instruments require digital single photondetection with either photomultipliers or expensive avalanchephotodiodes (APD) to achieve their performance, here we offer methods ofusing analog detection with conventional APD or silicon diodephotodetector operation to obtain excellent measurements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of the operational regions of the invention,exemplifying the approximate boundaries of particle size andconcentration for DLS/DDLS using the present invention. This exemplarysketch is for 168° backscatter, a visible volume of approximately 1nanoliter and a particle to liquid differential refractive index ofapproximately 1.2.

FIG. 2 is a system diagram, showing the modules that constitute a deviceused to make DLS/DDLS measurements.

FIG. 3 shows the illuminating and detection optics for a Small MatchedField configuration.

FIG. 4 shows a magnification of the configuration geometry of a SmallMatched Field centered at the plane entrance face of a colloidalspecimen in a rectangular container.

FIG. 5 shows the Small Matched Field configuration with a sessiledroplet.

FIG. 6 shows the Small Matched Field configuration with a wedgedspecimen chamber.

FIG. 7 shows the Small Matched Field with penetration into a circularvial.

FIG. 8 shows the illuminating and detection optics of a Large Field andAperture. One aspect of the present invention shows an illuminating coneof a much smaller angle than the receiving cone. The now larger diameterof the field stop may match the illuminated diameter at focus, or it maybe somewhat larger to observe a greater length of the beam focused wellwithin the colloidal suspension, which may have any of the geometriesshown in FIGS. 4 through 7, or otherwise.

FIG. 9 shows an exemplary plot of Signal versus Number of CoherenceAreas in a receiver aperture, illustrating a typical signal gainobtained by increasing the collection solid angle from well below (0.1)a conventional single coherence area to many such coherence areas (100).

FIG. 10 shows a Homodyne configuration, where a coherent light sourceemission surface, a scattering plane, a local oscillator image, and areceiver field stop are all optically conjugate, but not necessarily ofthe same radius.

FIG. 11 shows a Homodyne configuration with monitoring, extending FIG.10 to show apparatus whereby light source coherence and/or intensity maybe monitored during an experiment.

FIG. 12 shows an alternative Homodyne configuration, where a morecompact and simpler method of providing a local oscillator as in FIG. 10is enabled by a double pass through a specimen, while preservingcoherence over a receiver aperture.

FIG. 13 shows a Homodyne configuration for depolarization measurements,again exploiting the double pass approach of FIG. 12, but here drawnwith a 90 degree scattering angle for illustration purposes. Theoperation allows for amplification of either polarized or depolarizedscattered light, permitting measurements of both translational androtational diffusion.

FIG. 14 shows an Alternative Homodyne configuration for depolarizationmeasurements, again shown at 90 degree scattering angle for purposes ofillustration.

DETAILED DESCRIPTION

To maximize the measurable range of particle and particle suspensioncharacteristics, such as particle radius, aspect ratio, andconcentration, under a variety of empirical constraints, several opticalarrangements are presented. The applicable ranges of these arrangementsare sketched in FIG. 1, which is a diagram based upon numericalpredictions and approximate empirical verification of regions ofperformance capabilities accessible with the present invention. Theabscissa covers a range of fractional volume concentration of particlessuspended in a liquid, from 1 part in 10⁸ up to levels limited by themaximum feasible packing fraction. The ordinate covers a range ofparticle radii from 1 nm to 10 microns, rather larger than theconventionally accepted and typically more limited range of DLS. Thisexemplary figure is calculated and drawn for near-backscatter conditions(nominally 168°), for a particle-liquid differential refractive index ofabout 1.2 (polystyrene latex spheres in water), a visible sensing volumeof about 1 nanoliter, a coherent illumination of about 30 mW at 658 nmwavelength, and a collection solid angle of about 0.0034 steradians, orless where the signal would otherwise overload the detector, typicallytowards the upper right of the figure. This diagram shows severalimportant regions of interest, assuming plausible characteristics of asuitable detector and system noise from all sources. The region at theupper left (labeled ‘Too Few Particles’) corresponds to where thesensing volume contains an average of one particle or less. Althoughhaving less than a single particle present does not necessarily preventmeasurements, the experiment time increases rapidly and the intensityfluctuation, caused by particles leaving and entering the volume, canintroduce errors. The region at the lower left (labeled ‘Not EnoughSignal’) corresponds to a region where scattered light is inadequate toexceed system noise at a probably unduly pessimistic signal-to-noiseratio of unity. In the region at the upper right (labeled ‘MultipleScatter’) significant underestimates of the particle size arise fromlight being scattered more than once before detection. The centralbright region (labeled ‘Analog Backscatter DLS’) covers a rather benignspace where good data may be obtained without undue experimentaldifficulties by any of a number of pre-existing techniques, althoughtoward the edges of even this region increased care becomes essential.

In the remaining regions the range of measurements can be substantiallyimproved using one or more aspects of the present invention and thisapplies both to DLS and DDLS. The region labeled ‘Small Matched Field’may be measured with reduced multiple scattering errors by matching thediameters of the focus of the illuminating beam and the field stopimage. This is referred to as “Matched Field.” Reducing the diameter ofthe illuminating beam focus and the field stop image, typically to lessthan the order of the extinction length, is referred to as “SmallMatched Field.” In the region of FIG. 1 labeled ‘Large Aperture’, thereceiver, which in prior implementations is conventionally restricted toless than one or, less commonly, only a few speckles, or coherenceareas, as determined by the van Cittert-Zernike criterion, hereestimated to be about 7.6e⁻⁶ steradians, is increased to include manyspeckles (about 200 for this figure). This is referred to as “LargeAperture.” Measurement in the region labeled ‘Homodyne’ is achieved bycoherently mixing the scattered light with a coherent sample of thelight source as a homodyne amplifier.

To summarize, FIG. 1 shows how the measurement range may be extendedwith several optical configurations: 1) matching the illuminated andvisible volumes (Matched Field), the restriction to a field smallcompared with the extinction length (Small Matched Field); 2) areceiving aperture typically larger than a single speckle (LargeAperture); and 3) coherent homodyne amplification of the scatteredsignal (Homodyne). Note that the boundaries of the various regions areapproximate and created assuming analog detection, typically with anAvalanche Photodiode Diode. The use of photon counting instead of analogdetection can move the useful regions of the system down and to theleft, with consequent detriment to performance in the upper right.

Many applications of DLS and DDLS use photon counting rather than analogdetection. Although photon counting can typically move the usefulregions of the system down and to the left, measurements are seriouslycompromised for light levels leading to quantum detection rates greaterthan a few MHz. Note that the sharply drawn boundaries are, in fact,rather arbitrarily chosen levels of gently sloping functions of complexparametric variables.

FIG. 2 is a diagram of a DLS/DDLS colloid analyzer system. An OpticsModule 201 may contain various optical components that may include butnot be limited to, coherent light source, detector, lenses, detectors,apertures, shutter, mirrors, and polarizing components, and a containerfor the colloidal sample or specimen. A Coherent Light Source Module203, within the Optics Module 201, produces coherent light with suitablegeometrical properties and brings it to the Sample Module 205. TheCoherent Light Source Module may consist of a coherent light source andcollimating and converging lenses used to transmit and refract lightthat is presented to a colloidal specimen in 205; in some embodiments itmay also contain polarizing components, beam splitters, and/or mirrorsor other necessarily or desirable components. Light source wavelength,power, emission geometry, and coherence properties are design options.In one embodiment of the invention, the coherent light source may be asemiconductor laser (e.g., a Hitachi ML 120G21 laser); alternativeembodiments may include other types of lasers, e.g., gas, other solidstate, etc. In one embodiment of the invention, the lenses may beplastic (e.g., a 302-0380-780 from Optima Precision); alternativeembodiments may include lenses of glass or other materials of variousoptical and mechanical designs.

Coherent incident light 211 emerging from the Coherent Light SourceModule 203 enters the Sample Module 205, consisting of an adjustablesupport structure for the specimen container, which may be one ofseveral different geometries, e.g., a rectangular cuvette, circular orother shaped vial, wedge, capillary or droplet, or other method ofpresenting a free or constrained colloidal specimen. In some embodimentsof the invention, the Sample Module may also contain lenses, mirrors,beam splitters, and/or polarizing components. In one embodiment of theinvention, the Sample Module consists of replaceable or substitutablespecimen holders, which may comprise stationary or translatable slotsfor specimen containers of different geometries and/or opticalconfigurations/components. Variable position settings, consistent withtranslatable slots, optimize light penetration into the colloidspecimen. In another embodiment the Sample Module is a permanentlyplaced specimen holder. In still another embodiment, the Sample Modulemay be a specimen contained outside the instrument, as might beappropriate for in-line or in-situ measurement on a manufacturing line.In another embodiment, the specimen holder may contain a large number ofspecimens, which may be automatically presented in sequence, withappropriate selection of ideal operating conditions for each.

Scattered light 213 from the specimen enters the Detector Module 207,whose function is to transmit to the detector light scattered from awell defined region of the specimen contained in the Detector Module207. It contains converging and diverging lenses, aperture and fieldstops, and a detector; in some embodiments it may also contain otheroptical elements, such as polarizing components, beamsplitters orattenuators. The aperture stop controls the amount of light incidentupon the detector; a larger aperture increases the amount of lightdetectable from weakly scattering specimens, such as small particlesand/or low concentrations and/or small differential refractive index.The field stop, together with scattering angle and any confininggeometry, defines the visible sensing volume. In one embodiment of theinvention, the detector may be an avalanche photodiode, e.g., aPerkinElmer C30950E detector, converging and diverging lenses in thedetector module may be Part #22.1127 from Rolyn Optics, or othersuitable components. In one embodiment of the invention, severaldifferent aperture stops may be interchanged, for example by placingthem at different circumferential stations on a rotating wheel, or in asliding plate.

In another embodiment of the invention, a portion of the light from theCoherent Light Source Module 203, homodyne light 215, is directedthrough the Homodyne Module 209. The Homodyne Module may contain severaloptical components, such as lenses, mirrors, shutters, attenuators andpolarizing components, which deliver and control the amount andgeometrical configuration of coherent homodyne light incident 217entering the Detector Module 207, where it is coherently combined withthe scattered light 213 from the Sample Module 205 in a beam combiner,now included in the Detector Module. Homodyne light can amplify lowintensity scattered signals, increasing the signal-to-noise ratio, tosuppress the significance of unwanted optical flare, reduce measurementtime, and simplify analysis when multiple particle sizes are present.

The detector contained in the Detector Module 207 converts light to ananalog electrical signal 219, which is transmitted to a HardwareCorrelator or Data Acquisition Board 221 for processing and analysis.Processing may consist of correlating the acquired signal 219 to obtaina function from which particle translational and/or rotational diffusionproperties may be derived. Several signal processing options arepossible of which two are: 1) hardware correlation, where a speciallyprogrammed device (e.g., FPGA or DSP chip or other microprocessingengine) processes the signal in real time to create a small file of thecorrelation function that is relayed to the General Computer; or 2) theraw signal is digitized by a Data Acquisition Board, or streamed throughthe hardware correlator, to the General Computer for archive recording.This archived file may be processed immediately or later with moreversatility by one or more of a number of different methods. Latercomputer processing of archived raw data permits the flexibility ofdifferent signal processing options, such as software correlation, orother techniques operating differently on identical data. The resultsmay be compared and process choices later optimized, which is notpossible with the real-time non-conservative compression of raw dataimplicit in correlation. Processed or raw data 225 from the hardwarecorrelator/data acquisition unit is presented to the General Computer223 for computational analysis to determine particle size, sizedistribution, aspect ratio, and/or other quantities that can be foundfrom the data. Different embodiments of the invention may include ahardware correlator, data streaming to the computer, or a simultaneouscombination of both. Control functions for the correlator from theGeneral Computer or otherwise may also be passed through link 225, asmay instructions for reprogramming specific functions or properties ofthe processing capacity embodied in the Hardware Correlator or DataAcquisition Board.

The Coherent Light Source 203, Sample 205, Detector 207, and Homodyne209 Modules incorporated in the Optics Module 201, depicted in FIG. 2,are relevant to both DLS and DDLS operational modes. DLS measurements donot typically necessitate the use of polarizing components, as will beevident below, but polarizing components can sometimes improve DLSperformance. Polarizing components are necessary for DDLS measurementsand must typically be of high quality. Thus, some embodiments of theinvention may be used primarily for DLS measurement, while otherembodiments are used primarily for DDLS measurements, and still otherembodiments are used for both DLS and DDLS measurements.

FIG. 3 shows one configuration of the Optics Module 201, with theillumination and detection configuration for the Small Matched Field.The coherent light source 301 produces an illuminating beam diverging ina cone 303 that is typically somewhat larger than the aperture of thecollimating lens 305. (For beams whose intensity profile across anyradial section is approximately Gaussian, the boundary of such a cone isdefined at the beam half-width, i.e. the distance from the axis at whichthe electric field drops to 1/e of its central peak value, or where abeam's irradiance drops similarly to 1/e². It is convenient here toapply the same criterion even where the beam profile is not trulyGaussian, although other standards exist.) Although overfilling theaperture sacrifices a small amount of light, truncation of the beam bythe limb of the lens 305 modifies the light pattern at its focus in thespecimen. The circular symmetry of the beam is improved; the effect ofintrinsic astigmatism in the light source is partially suppressed, andthe intensity pattern at focus marginally more uniform than if the lensis allowed to accept the entire beam, as is the case for the otherimplementations described in this document, where the light source beammay be wholly transmitted by the aperture stop of the illuminatingsystem. Adjacent to and following the collimating lens 305 is aconverging lens 307; together they direct an illuminating beam in aconverging cone 309 to a focus in or near specimen 311, here showncontained in an inclined square cuvette 313. The illuminating beamincludes both the diverging illuminating cone 303 and the convergingilluminating cone 309, which may have the same cone angles, but neednot, and is incident on the specimen. The illuminating beam, comprisingthe available illuminating light, is typically spatially and temporallycoherent over the spatial extent of interest in the specimen and othergeometrical dimensions of the apparatus. Here and throughout thefollowing sections we follow the convention that ‘illuminated volume’refers to the volume within the specimen that receives light directlyfrom the light source, ‘scattering volume’ refers to the volume withinthe specimen that is illuminated by the light source AND light scatteredby the medium, i.e. multiple scattering, ‘visible volume’ refers to thevolume within the specimen that is visible to the receiving system,whether or not it is directly or indirectly illuminated, and ‘sensingvolume’ refers to the intersection region within the specimen common tothe illumination volume and the scattering volume. The diameter of thebeam at the collimated section between lenses 305 and 307, focal lengthof the focusing lens 307 and any residual aberrations, control bydiffraction the smallest possible illuminating beam diameter and theintensity contour of the focus within the specimen. A rectangularspecimen container, or cuvette, is shown in FIG. 3, but the descriptionsuffices for any specimen containment geometry, for which severalpossibilities are described below, although others may be envisaged.Particles in the scattering volume scatter light into the receiving cone317, only if it also lies within the visible volume, defined by apertureand field stops in the receiving system. The sensing volume common toilluminated and visible regions is thus optimally constrained to be mostsensitive to singly scattered light and least sensitive to multiplyscattered light, even though multiple scatter does occur everywherewithin the scattering volume. The sensing volume of the specimen 311 isfurther defined by the receiving optics comprising the collimating lens319, the aperture stop 321, and the converging lens 323, defining thereceiving cone 317, and the field stop 327, the mean scattering angle315, and the detector 329. Apart from any unwanted stray light frominevitable other properties of all optical systems, the only detectedlight is that scattered from the sensing volume into the receiving cone317, whose axis is at an angle 315, the mean scattering angle, to theaxis of the converging illuminating cone 309. The solid angle of thediverging receiving cone 317 is defined by the effective aperture stopof the receiving optical system, fairly well approximated by the leastdiameter of the collimating lens 319, the converging lens 323, andparticularly and ideally the stop 321, and its distance from the centerof the mean sensing volume. The projected diameter of the visible volumeis controlled by the field stop 327 of the receiving optical systemimaged into the specimen by the lenses 323 and 319, which may, but neednot, have the same focal lengths as each other. In FIG. 3, theilluminating and receiving beams are shown to be approximately the samesolid angle and focused just inside the colloidal suspension. Theillumination at focus and receiver field stop image planes are conjugateand radially coterminous at the center of the sensing volume. Thediameters of the illuminating beam forming lens 307 and the receivercollection lens 319, which are typically the same, physically constrainthe smallest feasible scattering angle 315. An exception may be made ifdifferent areas within a common lens are used; a situation lessdesirable however because of the difficulty of controlling coherentghost reflections and preventing coherent light from the light sourcescattering into the receiving aperture from other than the specimen.Scattered light in the converging receiving cone 325 is relayed to afocus at the field stop 327 by a receiver focusing lens 323. Thereceiving beam refers to both the diverging receiving cone 317 and theconverging receiving cone 325, which may, but need not, have the sameangles. Scattered light to be detected is contained within the receivingcone 325 projected beyond the field stop 327, where scattered light inthe receiving cone 325 diverges on to the surface of a detector 329,typically but not necessarily an avalanche photodiode whose diameterexceeds the diameter of the now diverging beam 325 at the axial locationof the detector, which is beyond the field stop. The sensing volumediameter must be as small as possible for minimization of thedetrimental effects of multiple scattering. The desirability of matchinga diffraction limited illumination beam diameter with the field stopimage diameter requires that the subtended solid angle of cones 309 and317, from lenses 307 and 319, be the same.

The Small Matched Field configuration in FIG. 3 refers to a situationwhere the illuminating beam and field stop image diameters are both thesame and simultaneously small enough at focus to be comparable to orpreferably less than the extinction length without physical interferenceof the lenses. Reducing the radius of the field stop image un-matchesthe illuminating beam and field stop image diameters, reducing both thevisible volume and the light received from the illuminated region, butreducing the singly scattered light disproportionately compared with themultiply scattered light. Increasing the radius of the field stop imagealso un-matches the beam diameters, increasing the visible volume butanalogously increasing the multiply scattered light disproportionatelymore than that from single scattering (see FIG. 8 for further discussionof these effects). A further extension of this idea is that theextinction length can be loosely associated on a statistical basis witha mean distance before a second scattering, indicating a typical sensingvolume dimension for the onset of multiple scattering. Restricting thescattering volume (and consequently the visible and sensing volume, seeabove) mean radius to be less than the extinction length reduces theerrors associated with multiply scattered light, which, if a significantamount is detected, gives a falsely low value for apparent hydrodynamicradius. A further and highly significant advantage is the choice ofpenetration depth, which may be optimally chosen, also on the basis ofextinction length. Empirically, good measurements result when theoptical penetration is nominally zero, but a slight increase or decreasein penetration where either less or more of the equivalent geometricallyvisible sensing volume lies within the specimen can be advantageous forextreme opacity or slightly greater transmission, respectively.

Four specimen containment configurations are considered in FIGS. 4through 7. A capillary containment is substantially similar to thatshown in FIG. 7. In each, the Small Matched Field condition is met whenthe illuminating beam diameter is equal to the field stop imagediameter. These specimen configurations can also be used for the LargeAperture and Homodyne configurations, referred to in FIG. 1.

FIG. 4, in one aspect of the present invention, shows an illuminatingcone 401, transmitted through an input wall 403 of rectangularcontainment geometry (cuvette) 405, into a specimen 407. The sensingvolume 409 is the region as formerly defined in which the illuminatingbeam 401 intersects the visible cone 419, but may also include multiplyscattered light from the whole scattering region which includes 409 and411. The location 417 at which the midpoint of the illuminating beamdiameter 415 and the midpoint of the field stop image diameter 413 meet,which is also their mutual point of best focus, defines the penetrationdepth, shown here in FIG. 4 as ‘zero’, at the inner face of the cuvette.The volume 411 is illuminated by multiple scatter, but is outside thesensing volume 409 that may also contain particles that contribute tomultiple scattering. The visible cone 419 is inclined to the axis of theilluminated beam 401 by an angle 421, the mean scattering angle. Notethat although it is not shown in this figure, the symmetry axis of thecuvette must be inclined slightly to the normal to the diagram toprevent reflection from any normal surface from introducingdestabilizing feedback into the light source. This condition must alsobe retained in all other methods of specimen containment.

In a colloidal medium, extinction length is a convenient quantitativedescription of turbidity, and may vary from tens of meters in a visuallyclear liquid, to a few millimeters for a milky appearance; less thanthis appears highly opaque. If the extinction length is significantlyless than the distance the sensing volume 409 extends into the specimen,multiple scattering can introduce errors. To minimize multiplescattering errors, in one embodiment, the sensing volume 409 should beas small as possible, but consistent with containing enough scatteringparticles and allowing adequate transmission to and from significantnumbers of particles.

The penetration depth into the specimen may be adjusted to optimize thescattered light signal and the simultaneous reduction in the amount oflight detected from multiple scattering. In one embodiment of thepresent invention, this adjustment may be performed via axialtranslation of the specimen. An example of a device for translating thecontainer that confines the specimen is a manually or electricallycontrolled micrometer. When in its chosen location, the specimen must berigidly stationary with respect to the apparatus. Optimization of thescattered light signal includes the largest signal compatible withrejection of multiple scatter and other parasitic effects, such ascoherent and incoherent stray light from any and all sources. Raytracing and geometrical compensation for each specific experimental caseallows optimization for different specimen opacities. In the case oftranslucent colloids, the primary intensity loss is by scattering awayfrom the incident direction. Note that in the configurations of FIGS. 3and 4, light that is scattered from more than one particle outside theilluminated volume is preferentially outside the visible cone 419 of thereceiver, with this condition improving as the extinction length becomeslarger compared with the incident beam diameter 415 and the field stopimage diameter 413, which are conjugate and coterminous at theintersection plane. FIG. 4 shows a conventional arrangement using arectangular cuvette with minimal penetration, shown with an intersectionpoint 417 at the cuvette wall, allowing for the best rejection ofmultiple scatter from scattering region 411. In practice in this andanalogously related configurations, the specimen container is inclinedout of plane to prevent any light reflected from the entrance surfacefrom entering the receiver cone. Surface contamination should also beminimized as a source of stray light, which can yield errors if itsdetected intensity is significant compared with the detected scatteredlight.

For extreme turbidity, where the extinction length is less than thesensing volume diameter, zero penetration of the intersection of thelight source and visible axes is optimal for the minimization ofmultiple scattering effects. Less than zero penetration, where theintersection point is outside the medium with only a small volume commonto illumination and visible cones, may be useful to reduce signaloverload at the detector. This situation, however, may or may notfurther improve the effects of multiple scattering, which areessentially controlled by the matching of the illuminating beam andfield stop image diameters at their overlap, but can also depend uponthe scattering properties of the specimen. Where the intersection pointis within the medium, for penetration greater than zero, the signalintensity at the detector can increase. This condition is favorable asturbidity reduces, but improves no further once the visible volume iswholly within the specimen. Further penetration can reduce signal bothfrom increasing optical aberrations and greater attenuation, but may bedesirable for highly transparent media where being further away from anyscattering from surface imperfections at the vial helps to reduce theeffects of coherent flare on the now much reduced signal, which couldcause an apparent increase of up to a factor of two in measured particleradius.

FIG. 5, another embodiment of the present invention, shows the SmallMatched Field configuration for a sessile droplet specimen. Incidentlight in the illuminating cone 501 reflects off mirror 503, istransmitted through horizontal flat plate 505, and enters specimen 507.The upper boundary 509 of the specimen defines the thickness and shapeof the droplet. The visible cone 511, inclined to the axis of theilluminating cone 501, lies out of the plane of the diagram in FIG. 5 atthe scattering angle, and the plate 505 is slightly inclined toeliminate reflection, either back into the illuminating source where itwould cause destabilization, or into the receiving cone where it wouldappear as excess noise or coherent flare. The illuminating beam diameterand the field stop image diameter, not explicitly labeled, are of equalsize for the Small Matched Field and are depicted at the entrance faceto the droplet. The position of the intersection of the beam diameterand the field stop image diameter is adjustable, and is shown here atlocation 513, the border of the specimen 507 and flat plate 505. FIG. 5shows the sensing volume 515 traversing the entire specimen and 517 asthe region outside the sensing volume that could contribute to multiplescattering. The illuminating beam enters the specimen from below in FIG.5; in other embodiments of the present invention the illuminating beammay enter the specimen at the upper boundary 509 of the specimen, orotherwise.

A sessile droplet specimen for DLS/DDLS has several advantages. Thespecimen can be deposited onto the flat surface 505 via a pipette,minimizing cost and labor associated with specimen container purchase,operation, disposal, cleaning, or contamination. Minimal specimen volumeis required for measurement of droplet specimens, reducing specimenmaterial usage and waste. Flat surface 505 may be an inexpensivemicroscope slide, which can simply be discarded after each use, orcleaned for reuse if appropriate.

FIG. 6, another embodiment of the invention, shows a wedged specimenchamber. The illuminating beam 601 enters a face 603 of the wedge, whichconsists of two inclined transparent flat plates 605 containing specimen607. The sensing volume 609 is controlled by the local wedge separationvia translation of the wedge and the dimensions of the field stop imagediameter, not shown. FIG. 6 indicates the wedge in an upright position,where translation would be in a vertical direction. Alternatively, thewedge may be configured to lie horizontal, where translation would be ina horizontal direction, with the specimen contained by sidewalk,capillary attraction, or otherwise. More generally, the wedge shapedcontainer is translatable to vary the sensing volume depth across thewedge. The wedge may be translated horizontally, vertically, or acombination of both. An example of a device for translating the wedge isa manually or electrically controlled micrometer. When in its chosenlocation the specimen must be rigidly stationary with respect to theapparatus. The mean axis of the receiving cone 611 is at an angle 613with the axis of the illuminating beam. Although the scattering angle isshown in the plane of the wedge angle, it could also be inclined atright angles or at any other angle if convenient. In the Small MatchedField, the diameters of the incident beam and field stop image, notdepicted, are equal at the point of intersection of their axes, which isshown in FIG. 6 as being close to the middle of the wedge. Penetrationis less important here than in the cuvette configuration because thesensing volume 609 typically encompasses the entire distance through thespecimen, which is varied in thickness by translation of the wedge alongthe direction of its taper. In principle this allows a sufficiently thinslice of specimen to be less than the extinction length.

The wedge is cheap and easy to manufacture, and can thus be a disposableitem. Several embodiments of the invention might include: a) differentmean wedge thickness to vary the amount of specimen in the container; b)different wedge angle to vary rate of change of thickness withtranslation; c) coatings on either or both of the inner wedge surface(s)to optimize wetting; d) coatings on either or both of the outer wedgesurface(s) to minimize reflection (reducing stray light) or optimizereflection (designing for a local oscillator on either the front or backsurface to significantly enhance the signal-to-noise ratio); e)absorbing or reflective coatings on either or both of the inner andouter surfaces of the rear plate, acting as a beam-dump; and f) manualor automated translation of the wedge to vary the sensing volume. Thislast item provides a distinct advantage over traditional vial or cuvettespecimen containment, offering a greater range of measurement capabilityover suspension concentrations, particle sizes, and differentialrefractive indices; high concentrations typically require small volumes,whereas large volumes yield greater sensitivity at low concentrations.The simpler optics of the wedge also allows quantitative optimization orcustomization for specific specimen types.

FIG. 7, another embodiment of the present invention, shows the SmallMatched Field configuration for a specimen contained within a circularvial, whose geometry is complex in that cylindrical refraction changesboth the sensing volume and the mean scattering angle with bothpenetration depth (horizontal) and translation (vertical) in the planeof the diagram, particularly if the vial axis is slightly inclined tothe normal of the diagram, as is usually required to prevent surfacereflections from entering either or both of the illuminating andreceiver cones. The illuminating beam 701 penetrates the internal wall703 of the vial 705 into the specimen 707. The region 709 is a volumeoutside the sensing volume 711 that may contain particles illuminated bymultiple scatter and that may contribute, albeit to a lesser extent, tomultiple scattering. The mean axis of the visible cone 713 is at aninclined angle 715 to the mean axis of the illuminating cone 701. Thelength 717 is the distance between the input wall 703 and theintersecting axes and focal planes of the illuminating and visiblecones, the so called penetration. Note that both the optical penetrationand the mean scattering angle vary slightly with physical penetrationbecause of refraction at the vial inner and outer surfaces. As withFIGS. 4 through 6, the intersection of the illuminating beam and thevisible cone defines the sensing volume 711, but the cylindricalcurvature of the inclined input wall complicates the refractivegeometry. Ray tracing and geometrical compensation for each specificexperimental case allows optimization for different specimen opacities.For example, optical distortions are minimal and sensing volume 711 ismaximized when the center of the volume coincides with the axis of thecircular vial, making both illuminating and received beams radial to thevial, but this cannot be achieved where extinction length is much lessthan the vial radius.

Four ancillary observations about variable penetration, variableaperture, and rejection of optical ghosts arise, but not necessarilyrespectively, from the four geometrical examples in FIGS. 4 through 7,and are further addressed in these descriptions below:

(1) The optimization of penetration, or the axial placement of thecommon focal planes of the illuminating beam and field stop images fromthe entrance face of the specimen, is highly desirable for differentcolloidal opacities. In opaque media, light scattered by particleswithin a field stop image close to the entrance surface of thescattering medium has less probability of multiple scattering beforedetection, making it the most effective place for the intersecting focalplanes of illumination and observing geometry. For more transparentmedia, a greater penetration not only increases the sensing volume butmitigates the effect of any surface contamination whose magnitude maynow become relatively greater than the amount of light detectable fromthe scattering particles. Although varying the penetration with thecircular vial has the same large advantages as those of the rectangularcuvette, some additional consequences must be accommodated. Scanning thevial with respect to the center of the intersection volume in air,either in the horizontal direction in FIG. 7 or at right angles(vertically in the plane of the diagram), moves the sensing volume andchanges both its dimensions and the mean scattering angle because of thecylindrical optical refractive power of the vial wall. An analogous butreduced and simpler effect also manifests in the other examples ofspecimen containment. Thus allowance must be made for the actual valuesof mean scattering angle and sensing volume center and dimensions,typically by ray tracing under the local empirically chosen conditions.There can be a small variability of the mean scattering angle,penetration, and aperture with local position within the sensing volume,but this is usually too small to introduce errors comparable with thosefrom other sources.

(2) When the penetration is minimal and where some high-opacity colloidsmay scatter so much light that the detector may be overloaded, thediameter of the receiver aperture stop may be reduced to prevent excesslight leading to detector overload and/or signal distortion. Reducingthe illuminating optical power electrically is undesirable, because itmay impair the stability of the light source and hence compromise thecoherence of the incident illumination. Attenuation by a neutral filterimplemented as a glass plate of the order of a millimeter or so thickmay also be undesirable because wherever in the beam it is installed, anormal placement can introduce destabilizing feedback from reflectionback into the coherent light source, while any inclination to mitigatethis changes the alignment geometry. Avoiding these effects byattenuation with a suitably coated and inclined pellicle is both lessrobust and more expensive. The reduction of a receiver aperture thatalready contains many coherence areas, while reducing the DC componentleading to overload, only reduces the AC component as the square root ofthe number of detected coherence areas and is thus the most desirablemethod of preventing overload or signal distortion. This is typicallyimplemented by exchanging receiver aperture stops of differentdiameters.

(3) Incoherent and unquantified coherent stray light must be kept to aminimum. This implies an uncontaminated specimen, well designed andchosen geometry and surface coatings, and cleanliness and freedom fromdamage of the specimen containers, particularly where the amount ofdetected scattered light approaches the noise floor of the system.Rejection of optical ghosts that could permit light reflected and/orrefracted by any surface or combination of surfaces in the system intothe receiving aperture is absolutely essential. Rejection of ghoststypically to third order may be acceptable, but possibly down to fifthorder for certain specimen containers. Tilting the plane of theilluminating beam entrance window to the specimen by about 5 degrees ofarc is typically effective as confirmed both by geometrical ray tracingand empirical visual and electronic observation through the detectionsystem. It may not always be possible nor easy to meet this requirement,for example, in the case of the upper surface of the sessile droplet.

(4) Where the scattered signal is small, for whatever reason, unknownamounts of coherent stray light lead to errors—typically giving afalsely large particle diameter by up to a factor of two. However, thismay be exploited for certain specimen containment devices and geometriesby deliberately introducing coherent stray light in excess of a few tensof times the power of the detected scattered light. This can emulate thehomodyne systems described below, and may be useful under certainconditions. The correlogram must now be analyzed as for homodyne toallow for the factor of two reduction in the apparent relaxation timescaling.

FIG. 8 shows another configuration of the Optics Module 201, showing theillumination and detection configuration for a Large Apertureimplementation. A coherent light source 801 produces an illuminatingbeam contained in diverging cone 803 that is smaller than the lens 805,which may have aspherical convex contours, and which focuses theilluminating beam into the specimen. The illuminating cone 807 formed bythe lens 805 is of smaller radius and lower convergence rate than thebeam formed by the collimating lens 305 and converging lens 307 in FIG.3, giving a larger focal waist 807, because of diffraction properties,and permitting the illumination of a much greater volume of specimen.The illuminating beam of light incident upon the specimen comprises bothdiverging cone 803 and converging cone 807. FIG. 8 shows a tiltedrectangular cuvette 809 containing specimen 811, but the discussionsuffices for other specimen geometries, for example those shown in FIGS.4 through 7. Some light scattered from the specimen 811, enters thediverging receiving cone 813 whose solid angle is defined by theeffective aperture stop of the receiving optical system, fairly wellapproximated by the least diameter of the collimating lens 319, theconverging lens 323, and particularly and ideally the stop 321, and itsdistance from the center of the mean sensing volume. The diameter of thefield stop 823 may now be chosen on the basis of the illuminating beamdiameter at the focus, typically but not necessarily to be the same. Ifit is the same, then this becomes identical with the Small Matched Fieldimplementation, but with an absolute diameter larger than that formerlydescribed—because the converging cone 807 has a smaller angle, leadingto a larger diffraction limited diameter at its focus. An alternativeembodiment of the present invention comprises an optical configurationof FIG. 3 with a large aperture stop 817. If the field stop imagediffers from that of the illumination at focus, the system becomes moresensitive to multiple scattering. Where the field stop image diameter issmaller than the illuminating beam diameter, the system becomes lesssensitive to total signal because the sensing volume is smaller thanbefore. Where the field stop image diameter exceeds that of theilluminating beam, the system sensitivity increases with the greatersensing volume of the increased visible illuminated intersection length.The focal lengths of the lenses 815 and 819 need not be the sameprovided that allowance is made for the image magnification between thefield stop and its image. Light scattered into the diverging cone 813 isrelayed to a focus at the field stop 823 by a receiver focusing lens 819forming a converging receiving cone 821. Beyond the field stop 823,scattered light continues to diverge at the same cone angle as 821,where it is incident upon the surface of a detector, which must be largeenough to intercept the entire scattered beam transmitted by the opticalaperture and field stops.

The configuration shown in FIG. 8 is well suited to the measurement ofnearly transparent specimens that are not too seriously constrained bymultiple scattering, and can be a good compromise to maximize the rangeof application for different extinction lengths. Of further advantage isthe increase of a field stop image diameter to equal or slightly exceedthe illuminating beam diameter (neither the field stop image diameternor the illuminating beam diameters are specifically indicated in FIG.8). Once the field stop image diameter exceeds the illuminating beamdiameter, further advantage may continue to accrue from a larger signalfrom the now longer sensing volume, more advantageous here because theilluminating beam attenuation is negligible over the distances ofinterest. The diameter of the aperture stop 817 may now be optimized interms of the available signal. A final doubling of the scattered lightsignal is available by translating the entrance wall away from the focalplane so that the entire optical intersection volume lies wholly withinthe specimen. Under this condition all dimensions of the sensing volumemust exceed the extinction length by enough to avoid the detection ofany significant multiple scattering—more readily satisfied for highlytransparent suspensions.

The logarithmic plot in FIG. 9 quantifies the advantage of theconfiguration found in FIG. 8 and elsewhere throughout thesedescriptions, achievable by detecting a number of coherence areas orspeckles greater than the conventionally accepted unity, as defined bythe van Cittert-Zernike coherence theorem. The collection solid angle inDLS is traditionally restricted to be less than one speckle to get thegreatest signal contrast or highest ‘intercept’ of the autocorrelationfunction of the detected signal, while satisfying necessary statisticalcriteria, but at the expense of an unnecessarily small optical signal.Including many coherence areas, while reducing both the contrast in thesignal and the proportional intercept of the correlogram, does howevergive a larger signal. Although the DC offset increases with the numberof speckle areas in the optical aperture, the AC signal risesapproximately only as the square root of the number of visible speckles.However this may conveniently be sufficient to overcome other noisesources while preserving and improving ergodic statistical integrity. Aformerly supposed disadvantage is that the larger solid angle includes aspread of scattering vectors and so reduces accuracy. However, this maybe acceptable for extremely small particles or low levels of scatteringwhere measurements may not otherwise be possible, particularly with theslower variation of the scattering vector with angle in nearbackscatter. For a given subtended normal radius of the illuminatedvisible specimen volume, increasing the aperture increases the number ofvisible speckles. At lower scattering angles, a vertical slit (or mask)can be used to minimize any spread in q-vector value while retainingsome of the advantages stated above. The solid angle subtended by eachcoherence area is proportional to the square of the quotient of thewavelength divided by the field radius of the virtual source at thesensing volume. In FIG. 9, the abscissa represents the number of visiblecoherence areas, which is proportional to the collection solid angle.The straight dotted line shows the arbitrarily scaled linear increase inDC level with aperture area, whereas the solid line represents thecorresponding detected AC signal, showing a linear improvement of signalas the detection area increases up to about a single van Cittert-Zernikecoherence area (one speckle), beyond which the signal becomesproportional to the square root of the number of speckles, according tothe statistics imposed by the central limit theorem. Note that althoughthe AC/DC ratio falls, the AC signal continues to increase, an advantagenot typically exploited by conventional DLS or DDLS systems, but ofsignificant advantage—about a factor of 20 for the example shown in FIG.9, when the DC is small enough not to overload the detector. While theelectrical output signal can be AC coupled, unfortunately this is notpossible for the optical input signal, preventing the advantageous gainfrom being even larger. The figure is sketched from approximations tothe rather complex mutual coherence functions and neglects both thediffraction limitation of the optics as small apertures are approachedand other optical subtleties, but the major conclusions are notmaterially affected.

Increasing the aperture is also useful for concentrated suspensions,where the sensing volume is matched to minimize the effects of multiplescatter. The small sensing volume typically used in the measurement ofconcentrated suspensions gives large speckles and the region in FIG. 9where the signal gain with aperture begins to increase more slowly thanlinearly, occurs at a larger solid angle. With high level signals, itis, however, also important not to exceed the permissible illuminatingpower density at which the specimen might be altered. The implicationsof this aspect of the invention are at least, but not necessarilylimited to, calculation of an optimal collection aperture, and thepossibility of obtaining sufficiently more signal to permit an avalanchephotodiode detector with analog electronics to replace a photomultiplierwith higher speed digital discrimination circuitry, thus avoiding errorssuch as dead time, pulse-pile-up and other limitations of high-powerdetection with a quantum realization (photon detection or counting)system. An aperture larger than the typical van Cittert-Zernike singlespeckle limit can thus be useful where signal level was formerlyinsufficient, regardless of whether the detection method is analog orquantum-realized. Conversely the reduction of collection solid angle towell below a single speckle carries no penalty where a smaller sensingvolume can postpone to higher concentrations the detrimental effects ofmultiple scatter, while simultaneously avoiding detector overload fromthe now much more highly concentrated and specimen that may scatter morelight.

FIG. 10 shows another configuration of the Optics Module 201, showing anillumination, detection, and homodyne configuration pertinent to the“Homodyne” referred to in FIG. 1. The coherent light source 1001produces a diverging illuminating cone 1003 transmitted wholly by theaperture of the collimating lens 1005, which is larger than the beam.After the collimated illuminating beam passes through a beamsplitter1007 and an optional attenuator 1009, a further lens 1011 produces aconverging cone 1013 focused to a suitable diameter at the specimen. Theilluminating beam, comprising the diverging illuminating cone 1003, thecollimated section (not labeled), and the converging illuminating cone1013 is incident on specimen 1017. The container 1015 for the specimen1017 is shown as square, but the description suffices for anycontainment geometry, e.g., as shown in FIGS. 4 through 7, or otherwise.The beamsplitter 1007 may be an uncoated flat glass plate with a wedgeangle that may be close to 1 degree of arc, or a little more, with thefirst surface reflection constituting the homodyne beam. The secondsurface reflection, not shown, with a comparable few percent of theincident power for an uncoated glass beamsplitter, is inclined away fromthe first beam to miss the following small lens 1029. The collimatedhomodyne beam is reflected from the first surface of the beamsplitterand proceeds through attenuator 1027, converging lens 1029, and pinhole1031, to be reflected by mirror 1033, proceeding then through attenuator1035 and collimating lens 1037 to beam combiner 1025. Scattered lightcontained in the visible cone 1019 and reflected by the plane mirror1021 is collimated by lens 1023 to pass through the beam combiner 1025,where it overlies the collimated homodyne beam arriving throughcollimating lens 1037 at approximately right angles to the front surfaceof the beamsplitter 1025. The scattered light and homodyne beam are bothreflected by mirror 1039, to proceed through aperture stop 1041 andconverging lens 1043. The scattered light and homodyne beam contained inconverging cone 1045 passes through field stop 1047 to detector 1049, asdescribed in earlier implementations.

Since the surfaces of beamsplitter 1007 are uncoated, their reflectionand transmission coefficients are given for either polarization by theFresnel equations relating refection properties to wavelength,refractive indices, and inclination angles. It is highly desirable tocollimate the beam before transmission through the inclined beamsplitter1007 to avoid distortion of the focal region by tangential and sagittalastigmatism, inevitable with a flat plate in an uncollimated beam, andyet more detrimental when that plate is not normal to the optical axis.It may also however be useful to allow small optical changes, such as,but not necessarily limited to, deliberate introduction of aberrationsto compensate for light source astigmatism, or other properties, ofadvantage to optimal implementation, and which will be obvious to thosewell versed in the art.

The beam combiner 1025 is also an uncoated wedged flat glass plate,which in the collimated beams introduces negligible aberrations into theimage of the field stop. Its wedge angle, which may be close to 1degree, is necessary to reject the contribution to the homodyne beamderived from the second or rear surface with respect to the homodynebeam at the beam combiner 1025. This means that that the angularseparation of the reflected beams gives the necessary spatial separationof the multiple homodyne source images, conveniently by more than twicethe sum of their radii at the field stop plane 1047, where only one beamconsisting of a combination of scattered and homodyne beams istransmitted by the field stop 1047. The second surface reflection wouldotherwise cause a partial overlap of two homodyne beams ofnot-too-dissimilar intensities, causing fringes that could be smallerthan the scattered coherence areas, and hence lower the modulation depthof the signal. It is also necessary to shift any returned image of therear surface reflected homodyne spot from the (potentially shiny) faceof pinhole away from the visible volume, or from the possibility ofbeing accidentally returned to the light source where its effects becomecomplex and detrimental because of potential modulation. The image atthe field stop 1047 of the homodyne light reflected from the frontsurfaces of the beamsplitters 1027 and 1025 is much smaller than thefield stop, through which it is aligned to pass. The collimated homodynebeam reflected from the front surface of the beamsplitter 1007 isfocused by the lens 1029 to a plane 1033 where its focal diameter isalso much smaller than the diameter of the incident beam 1013 in thespecimen. The light beam collimated by the lens 1037 is formed toimpinge upon the beam combiner 1025 at such an angle that its spatialcoherent wave-front perfectly overlies the mean wave-front of thescattered light in the receiving cone 1019. The homodyne beam diametersufficiently exceeds the aperture stop 1041 diameter so that itsintensity is sufficiently uniform for acceptable homodyne gain over theentire aperture, typically with a ratio of better than 2:1. The anglesare established such that following the beamsplitter 1025, the nominallyflat electric field wave-fronts from both homodyne and specimen areclosely parallel. The layout in FIG. 10 permits a more compactimplementation of the apparatus than is available where real timemonitoring is deemed to be necessary, as shown later in FIG. 11.

To assure the ability to choose optimal absolute and differentialintensities of the illuminating and homodyne beams, an attenuator 1009may be introduced into the collimated segment of the illuminating beam,where it not only avoids the introduction of negative sphericalaberration, but also prevents axial translation of the ‘disk of leastconfusion’ of the incident beam defining its best focus. The attenuatormay be implemented as a simple neutral density filter, of absorptive orreflective type, or as a rotating polarizer or other device capable oftunable attenuation. An analogous but adjustable attenuator 1035 may beintroduced for similar intensity control into the homodyne beam, butmust not change its apparent optical thickness with transmissionadjustment, to avoid axially translating the virtual source point of thehomodyne beam, since it is diverging at the station of the attenuator1035. Even without adjustment such a flat plate in a diverging beamintroduces negative spherical aberration, which here has almostnegligible detrimental consequences provided that it does not vary withtime. For discrete adjustment of intensity by the interposition ofattenuating plates 1027 of different thickness, these should be placedwhere the beam is collimated between the beamsplitter 1007 and thehomodyne focusing lens 1029. Since attenuators 1027 and 1035 arefunctionally equivalent, it is preferable to use only 1027 foraberration control, but this may be less convenient for physicalpackaging reasons. The orientation of the highly polarized (typicallygreater than 100:1) illuminating light source beam may be adjusted toprovide ‘S’ or ‘P’ or other polarization with respect to the plane ofthe figure, permitting a wide range of adjustment or adaptability ofrelative intensities by attenuators 1009 and 1027 or 1035, or Fresnelreflection at the beamsplitter 1007, the mirror 1033, or the beamcombiner 1025, or otherwise. For small particles, typically in or closeto the Rayleigh scattering regime, the polarization orientation of theincident beam does not have a strong effect in near-backscatter, but canbe a useful variable for independent control of the relative beamintensities.

To exploit the advantages of homodyne gain, two intensity conditionsmust be satisfied; (1) the total signal must not overload the detectornor exceed its linearity over a usable range, and (2) the ratio ofhomodyne to scattered light intensities must be large enough, typicallygreater than fifty times, to avoid introducing excessive relaxation timemeasurement errors, which would otherwise arise because the relaxationtime without homodyne is typically half that measured with an excessadditive coherent field. To satisfy these conditions simultaneously fordifferent amounts of scattering, it is desirable to control the homodyneintensity independently of the illumination of the visible volume.

Many applications provide optical homodyne gain and increasesignal-to-noise ratio by mixing with the scattered light a spatially andtemporally coherent electric field wave-front derived from the sameoptical source. The maximum possible theoretical improvement is wherethe residual noise is the shot noise implicit in quantum detection ofthe scattered light, but in practice is usually limited by the noise inthe illuminating light source. The critical interferometric alignmentnecessary to assure that mutual spatial coherence is maintained over thereceiving aperture may be relaxed to produce a simple and robustapparatus, as described in FIG. 10.

The first salient feature of this homodyne implementation is that thelight source emission surface, its focal plane in the medium, the focusof the homodyne beam 1031, and the field stop 1047 are all opticallyconjugate, although not all illuminated patterns are of the same radius.The second is that the diameter at the focus of the homodyne beam 1031is much smaller than the field stop 1047 so that small misalignmentswill not significantly affect transmission by the field stop nor theeffectiveness of the wave-front overlap. Typically, for optical homodynegain, the wave-front match over the common area has a phase disparity ofsubstantially less than π/2. This remains true if the receiver aperture1041 contains less than one van Cittert-Zernike area, or more simply butless exactly, a single speckle. Where the aperture contains manyspeckles, a condition attractive to increase the available signal, thephase match over each coherence area (speckle) has the same π/2condition. However, the homodyne phase front need not be constant overthe entire aperture provided that it satisfies the coherent interferencecondition over more than each scattered speckle diameter independently.This relaxation of the interferometric alignment condition is possiblebecause the dynamically random phases of the scattered light from anensemble of independent particles assures proper temporal modulation,with typically, but not necessarily, Gaussian statistics. Theimplementation introduced in FIG. 10 allows homodyne light of acceptablyuniform intensity and phase coherence over the receiving aperture to beguaranteed with more relaxed and forgiving alignments than conventionalinterferometric requirements. Even if the π/2 condition is compromisedthe detriment to the signal falls only slightly.

Homodyne amplification offers advantage in both low and highconcentration applications. First, where the particle suspension ishighly transparent and only a small signal is available, because theparticles are small, at low concentration, or with a refractive indexclose to that of the liquid, the homodyne beam may be used to amplifythe scattered signal power. By increasing the homodyne beam power, theamplification may be made as large as desired until the light sourcenoise exceeds that of the shot noise in the signal up to the limit ofemission noise, beyond which no further improvement is obtained. Evenbefore the onset of this condition, shot noise in the signal may limitthe accuracy or even feasibility of experimental measurements. In thisfirst condition of low scattering, the sensing volume may be usefullyincreased to provide more particles, and the receiving aperture may nowbe expanded beyond the often accepted single speckle solid angle withadvantages discussed elsewhere in this document. Second, for highlyconcentrated and turbid particle suspensions, the homodyne system offersadvantages in entirely different ways. The illumination and receivermatching conditions are necessary as before, but as the volume boundedby the projected area is reduced the sensing volume contains fewerparticles, yielding a reduced signal, with potentially changedstatistics, as the intensity fluctuation spectrum may distort the phasespectrum indicative of the particle size. Increasing the light sourcepower may increase signal, but because of the small field this raisesthe intensity or power density, perhaps beyond the damage threshold ofthe specimen. Homodyne gain is now useful to raise the signal as thesensing volume is reduced beyond formerly accessible limits, but shiftsfrom the conventionally statistically stationary measurement to theequivalent of an intensity fluctuation spectrum, more difficult tointerpret but now possible to analyze. A second advantage in this casemay be less obvious. At extreme concentration, particles may eitherstick to the wall or be otherwise constrained in their assumed isotropicBrownian motion. The light from these adds coherently to the desiredscatter but changes the apparent relaxation time behavior leading to afalse size estimate. With added homodyne this light can be swamped intoinsignificance with reduction of associated errors up to higherconcentrations, typically by the reduction of cross-terms in thescattering matrix.

As is well known to those versed in the art, homodyne amplification hasmany potential advantages in addition to those presented above. Itdilutes and hence reduces the detrimental effects of any coherent, oreven incoherent, stray light. It also produces an undistortedmeasurement even when the statistics of scattered light are notGaussian, as is traditionally assumed. The only accessible conventionalmeasurement is that of the second order, or intensity, the light sourcenoise exceeds that of the shot noise in the signal up to the limit ofemission noise, beyond which no further improvement is obtained. Evenbefore the onset of this condition, shot noise in the signal may limitthe accuracy or even feasibility of experimental measurements. In thisfirst condition of low scattering, the sensing volume may be usefullyincreased to provide more particles, and the receiving aperture may nowbe expanded beyond the often accepted single speckle solid angle withadvantages discussed elsewhere in this document. Second, for highlyconcentrated and turbid particle suspensions, the homodyne system offersadvantages in entirely different ways. The illumination and receivermatching conditions are necessary as before, but as the volume boundedby the projected area is reduced the sensing volume contains fewerparticles, yielding a reduced signal, with potentially changedstatistics, as the intensity fluctuation spectrum may distort the phasespectrum indicative of the particle size. Increasing the light sourcepower may increase signal, but because of the small field this raisesthe intensity or power density, perhaps beyond the damage threshold ofthe specimen. Homodyne gain is now useful to raise the signal as thesensing volume is reduced beyond formerly accessible limits, but shiftsfrom the conventionally statistically stationary measurement to theequivalent of an intensity fluctuation spectrum, more difficult tointerpret but now possible to analyze. A second advantage in this casemay be less obvious. At extreme concentration, particles may eitherstick to the wall or be otherwise constrained in their assumed isotropicBrownian motion. The light from these adds coherently to the desiredscatter but changes the apparent relaxation time behavior leading to afalse size estimate. With added homodyne this light can be swamped intoinsignificance with reduction of associated errors up to higherconcentrations, typically by the reduction of cross-terms in thescattering matrix.

As is well known to those versed in the art, homodyne amplification hasmany potential advantages in addition to those presented above. Itdilutes and hence reduces the detrimental effects of any coherent, oreven incoherent, stray light. It also produces an undistortedmeasurement even when the statistics of scattered light are notGaussian, as is traditionally assumed. The only accessible conventionalmeasurement is that of the second order, or intensity, correlationfunction. To obtain the required first order, or field, correlationfunction from which the Doppler spectrum may be recovered, the Siegertrelationship is conventionally assumed, but this relationship is onlytruly correct for Gaussian statistics. Homodyne amplification makes thefirst-order correlation function directly accessible and obviates theneed for questionable statistical assumptions. Where two or moreparticle sizes are simultaneously present the cross terms are alsosignificantly reduced, making the recovery of different particle sizesboth easier and more accurate. Since the application of homodyne alsoreduces the measured relaxation frequency by a factor of two, a reducedsystem bandwidth can similarly reduce noise.

FIG. 11 shows a homodyne arrangement similar to FIG. 10 with theaddition of real time monitoring facilities, desirable to assure that atall times during the experimental measurements the conditions wereactually as initially and/or finally supposed. As in every examplediscussed so far, the temperature of the coherent light source 1101 iscontrolled by a Peltier bismuth telluride thermoelectric stack cooler1103, which is monitored in a closed loop servo by a thermistor 1105. Asin FIG. 10, the illuminating beam is collimated by the lens 1109, and asample is extracted by the beamsplitter 1111. Beyond this point theremaining illuminating optics, sample module, and detection optics, andeven most of the homodyne optics are the same as those shown in FIG. 10,except for a change of physical placement to accommodate differentpackaging, and will not be discussed further here. A monitoring beam1131 is reflected from the rear surface of beamsplitter 1111, inclinedto the front surface of beamsplitter 1111 by a small angle (typically 1or 2 degrees), and thereafter diverges from the homodyne beam 1141. Thecollimated beam 1131 monitors the power and/or the coherence of thelight source output. The beam transmitted through the interferometer1133 impinges upon a detector 1135, which may be a silicon diodesufficiently large to intercept the entire beam and which reports viaboth DC and AC coupled electronics, the optical power, and anymodulation of the sampled light source beam. The thin flat plateinterferometer 1133, which may be a small piece of a microscope slide orcover slip, reflects similar intensities from both itsnot-quite-parallel surfaces. The two similar beams partially overlapsufficiently to produce interference fringes at infinity or elsewhere.Beyond a neutral absorber or rotating polarizer 1137, a monitoring CCTVcamera 1139 observes a suitably attenuated image of these fringes, whosehigh apparent contrast indicates the retention of coherence in the lightsource. The ideal appearance is of sharply contrasting regions appearingstationary and of constant intensity. Three effects may be usefullymonitored by this mechanism. (1) Should the light source mode hopbecause of an inappropriate combined choice of light source drivecurrent and operating temperature, controlled by the thermoelectricPeltier cooler 1103 and monitored in a closed loop by the thermistor1105, light source phase jumps will reverse the contrast of the fringes,with the image appearing to skip at right angles to the fringeorientation, or with variable loss of fringe contrast. This condition ofunstable light source operation on the boundary between coherent modesmay be remedied by slight adjustment, either of light source current ortemperature, to place the operating point centrally between adjacentmode transitions. (2) Should the light source receive any coherentfeedback from instability within the optical system, the fringes mayperhaps shimmer at the mechanical resonant frequency of sources of theparasitic reflection. As is well known in the art, this and any acousticsensitivity effects must be mitigated by avoidance of significantoptical ghosts. (3) Feedback to the light source can arise from particlescattering itself, causing a randomly fluctuating fringe blur andproducing parasitic modulation on the homodyne beam. The backscatterfrequency modulation of the light source is similar but not identical tothe modulation of the scattered beam by direct scattering from thespecimen into the receiver. Where the homodyne amplification gain isapplied to densely scattering specimens this effect must be allowed for.With certain extreme specimens, such as large, or large numbers of,retro-reflective particles, an error boundary at lower concentration maybe introduced that might not otherwise have been suspected, making theindependent monitoring in real time a useful feature for certainapplications. The parasitic modulation of the light source bybackscattered light is of a slightly higher frequency than the desiredlight at the mean scattering angle by the amount determined by thescattering angle (or conventionally ‘q-vector’), and may be compensatedduring analysis by knowing the relative intensities in the presentimplementation and under the specific operating conditions during themeasurement.

The monitoring portion of the configuration shown in FIG. 11, i.e. beamsplitter 1111, interferometer 1131, detector 1135, neutral absorber orrotating polarizer 1137, and CCTV monitoring camera 1139, can also beused in the configurations shown in FIGS. 3 and 8. When adding abeamsplitter to the configurations, such as found in FIGS. 3 and 8, thebeamsplitter is positioned in front of the coherent light source tosplit the illuminating beam. In general, a slightly wedged beamsplitter(depicted in FIGS. 10 and 11) splits an incident beam into three beamscomprising a beam reflected from the first face of the beam splitter, abeam reflected from the second face of the beam splitter and refractedtwice by the first face, and a twice refracted beam transmitted throughthe beam splitter. Higher order reflections are either insignificant inpower or directed away from later components or both. Either of the tworeflected beams may be used for monitoring the illuminating beamintensity and coherence, but it is most convenient and optically optimalto use the refracted reflection from the second surface of thebeamsplitter for monitoring, allowing the more aberration free firstsurface reflection to become the homodyne beam.

The implementations of FIGS. 10 and 11 allow a particularly simplealignment sequence of lockable adjustments and stable retentionthereafter. The following sequence uses the labeling of FIG. 11 butapplies similarly to FIG. 10, and elsewhere:

(1) Light Source Collimation: Action; translate the lens 1109 withrespect to the light source emission surface. Criterion; beam diametermeasured by a ruler is the same just after the lens 1109 and severalmeters beyond.

(2) Receiver Collimation: Action; translate the field stop with respectto the lens 1159 while transilluminating the field stop with anotherlight source of the same wavelength and with the mirror 1155 removed.Criterion; beam diameter measured by a ruler is the same just after thecollimating lens 1159 and several meters beyond.

(3) Receiver Focus: Action; replace the mirror 1155 and translate abrushed aluminum plate through the image of the transilluminated fieldstop. Criterion; scattered speckles appear as large as possible at thebest focus where the beam is also smallest and most symmetrical.

(4) Illumination Focus: Action; with the brushed aluminum plate fixed atthe best focus of the receiver lens 1127 as above, translate the lens1115 to adjust axially the light source image to coincide with the bestfocus of the receiver. Criterion; scattered speckles appear as large aspossible at the best focus of the light source, where the beam is alsosmallest and most symmetrical.

(5) Source Field Overlap: Action; with the brushed aluminum plate fixed,and the transillumination of the field stop removed, steer the angularposition of the mirror 1155. Criterion; scattered light is maximallytransmitted by the field stop 1163.

(6) Homodyne Focus: Action; axially translate the homodyne focusing lens1147. Criterion; the homodyne beam uniformly overfills the aperture stop1157 at its largest opening.

(7) Homodyne Collimation: Action; translate the homodyne collimatinglens 1153. Criterion; beam diameter measured by a ruler is constant justafter the lens 1153 and several meters beyond. The beamsplitter 1129need not be removed since it transmits sufficient light to permit theundistorted measurement of collimation.

Note that (6) and (7) may require iteration to meet both criteriasimultaneously.

(8) Homodyne Overlap: Action; radially translate (typically using asprung and lockable grease plate with orthogonal translation screws, orotherwise) the homodyne focusing lens 1147. Criterion; the homodyne beamis maximally transmitted by the field stop 1165, and other reflectedghosts are blocked.

Finally, the detector 1165 is placed sufficiently close to the fieldstop to assure that all light transmitted by the aperture 1157 and field1165 stops is detected by the sensitive area of the detector. Lesscritical placements, for example of the attenuators or the monitoringdetector CCD camera, are trivial; but minor adjustments repeating theabove sequence may improve signal and homodyne overlap prior to thenecessary locking of all adjustments.

FIG. 12 shows an alternative configuration for implementing the homodynegain, with simpler and fewer critical alignments and greater stability,especially benefiting specimens whose extinction length well exceeds thespecimen container breadth, e.g., vial diameter. As with all otherimplementations described here, the temperature of the coherent lightsource 1201 is controlled to place the operating point near the middleof the stable region between two adjacent mode transitions by a Peltiercooler 1203, which is monitored in a closed loop by a thermistor 1205.The coherent light source 1201 produces a diverging illuminating cone1207 that is wholly intercepted by the aperture of a convex aspheric orother lens 1209. FIG. 12 shows a circular vial 1211 containing specimen1213, but the discussion applies to other specimen geometries where suchtransmission may be arranged. Light transmitted through the specimenbecomes the homodyne beam, unlike the configurations in FIGS. 3, 8, 10,11, and later 14, where homodyne light bypasses the specimen in analternative optical path. The transmitted beam is reflected from thefront surface of a suitably coated plane mirror 1215, sufficiently thickthat the reflection from its rear surface, should that have significantpower, is also dumped with the main transmitted beam. A secondreflection from the front surface of a suitably coated mirror 1217further attenuates the beam while returning it to the receiver lenstrain axis 1239. Three more lenses, a focusing lens 1219, which becauseof its necessary speed may be aspherical, followed by a collimating lens1221, and a focusing lens 1223, configure the homodyne beam 1227 tooverfill the receiver aperture 1233, assuring a reasonably uniformhomodyne spatial intensity distribution. Scattered light enters thediverging visible cone 1225, passes through the lenses 1229 and 1231,the aperture 1233, and the field stop 1235, which is smaller than thehomodyne cone 1227 and wholly contained within it. Eventually, both thehomodyne beam and the scattered light contained in the divergingreceiver cone 1225, overlie each other almost perfectly and reach thesurface of the detector 1237 together.

The essence of this alternative homodyne scheme is that the homodynebeam actually passes twice through the specimen. Near forward scatteredlight from both passes of the transmitted homodyne beam can beproportionally large for predominantly forward scattering particles,typically around 1 micron radius or a bit less. It does not howeverimpair the DLS measurement for two reasons; (1) it is centered on zerofrequency, appearing no more than a slight phase change on theunmodulated homodyne beam, and (2) it is also typically alreadyattenuated to about four orders of magnitude below that of the incidentbeam, from which the required near back scattering arose. However, thehomodyne beam intensity is sufficiently larger than the near backscatterso that its amplification is effective and valuable. Since here homodyneis only relevant to transparent specimens that scatter little light, theilluminating beam is transmitted with negligible attenuation.

While it is only necessary that the spatial coherence area for thehomodyne beam shall be larger than each van Cittert-Zernike coherencearea for the scattered signal beam, here a single speckle of thehomodyne beam can readily fill the entire receiving aperture, even whenaberrations are present. One desirable consequence of the normallyunforgiving Lagrange Invariant is that the homodyne beam is much smallerthan the cross section of the illuminated specimen volume at the commonpoint represented by the center of the sensing volume, the center of thevial, and the conjugate image of the field stop. This has some advantagefor alignment since the smaller focal diameter may lie anywhere withinthe conjugate field stop image without detriment. Because the beam isnow so highly attenuated, even when more tightly focused, it issufficiently below any intensity which could affect the specimen,especially in the case here of low absorptivity guaranteed by specimentransparency. All the fixed homodyne advantages described above are thusavailable in a much simpler apparatus. Furthermore, the added homodynecomponents, 1215, 1217, 1219, 1221, and 1223, can be integrated as aspecialized specimen holder in a Sample Module, allowing possiblesubstitution for other specimen holders such as those discussedpreviously or others.

FIG. 13 extends the ideas of homodyne amplification of nearbackscattered light to amplification of light scattered at other angles,of which an arrangement at 90 degrees is shown. This configurationoffers the possibility of making measurements of the rotationalrelaxation rate, from which an estimate of particle aspect ratio may befound. The measured frequency of translational relaxation rate dependsupon the sine squared of the scattering angle (the q-vector), whereasthe frequency of the rotational diffusion rate is independent of angle.This has permitted good measurements of rotational relaxation rate bymeasuring the frequency spectrum of the depolarized forward scatter,which is coherently amplified by the unscattered, but typically phaseshifted, coherent illuminating beam, using suitable polarizationselection, e.g., as described in Digiorgio, et al., Forward depolarizedlight scattering: heterodyne versus homodyne detection, Physica A, 235,279, 1997. The former arrangement does not simultaneously, nor easily,measure the translational relaxation rate, from which mean hydrodynamicradius may be found. The extension shown here, typically, but notnecessarily, at right angles to the illumination, allows for measurementof both translational and rotational relaxation spectra.

In FIG. 13, fairly polarized (say 100:1) coherent light from theilluminating source 1301 is slightly focused by an aspheric collimatorlens 1303 and rendered of much higher polarization purity (say 1e7:1) bya high quality polarizer 1305 aligned so that the ‘P’ polarizationvector is normal to the plane of the diagram. The use of such apolarizer in an only slowly converging beam does not degrade itsperformance below what is acceptable. The incident beam passes throughthe specimen 1307 in a specimen container 1309 shown in FIG. 13 as acircular vial, but the discussion suffices for other specimengeometries, as discussed previously. The specimen container whose axisis slightly tilted to prevent optical ghosts from reaching the detectorshould be free from any significant optical birefringence. Thetransmitted light becomes a slightly diverging homodyne beam, which iscollimated by a lens 1311, redirected by mirrors 1313 and 1319, andincreased in diameter by a telescope consisting of short 1315 and longer1321 focal length lenses, before being redirected by mirror 1327 andrefocused by lens 1329 through the center of the earlier illuminatedspecimen. Beyond the second pass of the specimen, the homodyne beamcontained in homodyne cone 1331 continues to diverge until refocused bythe receiver achromatic lenses 1333 and 1339, passing through theaperture stop 1337 and a field stop 1341. The divergence of thishomodyne beam is sufficient that the illumination over the detector isfrom a single speckle or van Cittert-Zernike coherence area with asufficiently uniform intensity to act as an excess local oscillator. Thethree plane mirrors 1313, 1319, and 1327 are of sufficient thickness todump second and higher order reflections from the detector field ofview, and their surface coatings reflect a suitably attenuated beamwithout significantly altering the polarization—typically an attenuationfactor of 1e4 is sufficient to prevent detector overload (e.g., 1microwatt for an exemplary avalanche photodiode detector, APD, such asthe PerkinElmer C30950E).

Before the second high-quality polarizer 1325, itself necessary torestore the polarization ratio, is a rotatable half-wave plate 1323 totune the intensity and polarization sufficiently that, when combinedwith the mirror attenuation, the residual light is of an intensity,diameter and polarization sufficient to act as a homodyne localoscillator to amplify the depolarized light formerly scattered in thedirection of the detector 1343 during the first pass, withoutoverloading the detector. This semi-dark field is approached as thepolarizer 1325 aligns with the ‘S’ polarization vector. The third highquality polarizer-analyzer 1335 can be oriented to accept ‘P’ scatteredlight from the initial input beam whose frequency is dependent ontranslational particle motion, or ‘S’ polarized to accept the amplifieddepolarized component from the first transit of the cell. In the firstcase of translationally sensitive ‘P’ polarized scatter, a shutter 1317can be closed and the apparatus will operate in the conventional senseto obtain translational information. Alternatively, opening shutter 1317in ‘P’ polarization mode allows homodyne gain for translationalrelaxation measurements, similar to the configuration in FIG. 12. Notethat for particles small enough to be in the Rayleigh regime, the ‘S’polarized scatter from the ‘P’ polarized incident beam is vestigiallysmall and can be further extinguished by setting the analyzer 1335 for‘P’ transmission.

For Rayleigh particles in the rotationally sensitive case, where theshutter 1317 is closed and the analyzer 1335 is set to accept thedepolarized ‘S’ scattered component, the signal is usually both muchsmaller and typically of a higher frequency than that of the ‘P’scattered beam. Opening of the shutter 1317 allows for homodyneamplification of the 90 degree scattered depolarized component by mixingwith the transmitted beam of suitable polarization and coherence. Thehalf-wave plate 1323 and polarizer 1325 accomplish three tasks: 1) theyblock forward scattered depolarized light emerging after the first passthrough the specimen; 2) they rotate the transmitted ‘P’ polarized beamto ‘S’ polarized; and 3) they can be used to further adjust thetransmitted beam intensity beyond the 1e4 attenuation provided by themirrors to avoid overloading the detector. Analyzer 1335 rejects the ‘P’polarized forward scattered depolarized light after the second pass (‘P’polarized because it enters the specimen as ‘S’ polarized) and allowsthe ‘S’ polarized transmitted beam, of suitable polarization andcoherence, to mix with and amplify the depolarized light scattered at 90degrees. As is well known in the art, small adjustments of the rotationof the various polarizing components can adjust relative beamintensities for optimal signal and signal-to-noise ratio. Naturally, alloptical components must be as free as possible from parasitic opticalbirefringence. While this arrangement is easily implemented, the qualityof components must be high. An alternative arrangement is shown below inFIG. 14, together with further descriptions which may apply equally toeither configuration.

FIG. 14 shows a configuration capable of functionality analogous to thatin FIG. 13, but with slightly different properties that may haveadvantages for alignment, polarization selection, purity of coherence,beam uniformity, and component quality. In FIG. 14 the homodyne does notpass through the vial a second time but bypasses the specimen chambercompletely. This markedly reduces the need for optical quality in thespecimen container, particularly freedom from birefringence, whileretaining some of the alignment tolerance. Indeed, a misplacement evengreater than that formerly allowed is now acceptable as long as threeconditions are satisfied: (1) the polarization of scattered and homodynebeams is the same; (2) the intensity of the homodyne beam varies littleover the aperture; and (3) the typical dimension of each vanCittert-Zernike coherence area in the homodyne beam is larger thanspeckles from the scattering medium. Slight impairment of the thirdcondition is not catastrophic, but reduces the contrast, i.e. theintercept, of the correlogram

In FIG. 14 the coherent light source 1401 produces a beam predominantly‘P’ polarized normal to the diagram. A lens 1403 focuses the beam to thecenter of the specimen 1409, which it reaches after traversing thepolarizing beamsplitter 1405 and polarizer 1407. Beamsplitter 1405reflects most of the ‘P’ polarized light towards the specimen 1409, fromwhere the light scattered near 90 degrees is collected by the receivingsystem cone 1411, beyond which it is collimated by the lens 1413 andpassed through a polarizer 1415 that may be rotated to transmit either‘P’ or ‘S’ polarized light only. Similar to the configuration in FIG.10, the homodyne beam emerges ‘P’ polarized from the beamsplitter 1405,where most of the intensity has been reflected towards the specimen. Thepolarization of the homodyne beam is rotated by the half-wave plate 1419to be either ‘P’ (half-wave plate axis parallel to incident ‘P) or ‘S’(half-wave plate axis inclined at 45 degrees to incident ‘P), focused bythe lens 1421, redirected by the mirror 1425, and recollimated by thelens 1429. Near the focus between lens 1421 and mirror 1425 is a shutter1423, which may be used to block the homodyne beam when that conditionis desired (primarily for translational measurement of specimens thatscatter sufficient light). The homodyne beam transmitted by thebeamsplitter 1417 is absorbed by a beam dump (not shown). The homodynebeam, whose illumination boundary is defined by homodyne cone 1427,drawn larger than the limits imposed by the lens 1429 in FIG. 14,overfills the aperture 1435 by enough to satisfy a criterion ofsufficiently uniform homodyne gain. When the homodyne beam polarizationis ‘S’ oriented, typically used with the polarizer 1415 similarly ‘S’oriented, only the depolarized rotational spectrum is amplifiedsufficiently to be well-measured. Scattered light 1411 is combined inthe beamsplitter 1417 with the homodyne light, a coherent sample of theilluminating beam contained within an illuminating cone 1427, passedthrough the ‘S’ oriented polarizer 1431 to remove any residual ‘P’polarization effects, to converging lens 1433, through aperture 1425 andfield 1437 stops to detector 1439.

The configurations in FIGS. 13 and 14 may be operated in any one of fourmodes, summarized in the table below with approximate performanceestimates. In all cases the input polarizer 1407, whose purpose is toimprove the light source polarization ratio in the sample illuminatingbeam, is ‘P’ oriented. The first two modes are used to measure thetranslational relaxation rate. In one mode the shutter 1423 is closed(no homodyne amplification), in the other it is open (homodyneamplification). In both modes the half-wave plate 1419, the outputpolarizer 1415, and the analyzer 1431 are all oriented for ‘P’polarization transmission. The third and fourth modes, again with openor closed shutter 1423 (corresponding to no homodyne and homodyneoperation, respectively), allow measurement of the rotational relaxationrate, where the half-wave plate 1419, the output polarizer 1415 and theanalyzer 1431 are all oriented for ‘S’ polarization transmission.

For an exemplary estimate of performance, consider a suspension of 10 nmradius particles with a differential refractive index of 1.2, at avolume fractional concentration of 1e-4, exposed to a 658 nm wavelengthillumination power of 30 mW. The measurement is made at 90 degrees,where scattering efficiency from spheres is about 1e-3 sr⁻¹ for ‘P’ andabout 1e-9 sr⁻¹ for ‘S’ (the Rayleigh ‘dip’). With a 50 mm focal lengthcollection lens, the collection solid angle can be as high as 0.0025 sr;where a 100 micron field stop image diameter yields about 200 specklesor coherence areas. The homodyne beam, when present, has a power ofabout 1 microwatt, sufficient not to overload a typical AvalanchePhotodiode Detector, APD such as the PerkinElmer C30950E, whose noiseequivalent product (NEP) may be as low as 5e-14 W/√Hz. In an analogdetection mode at a gain of 5e5 V/W, the overload condition is typicallyextended with active DC compensation of the transimpedance amplifier toa few volts. Assuming particles are sufficiently ellipsoidal to give arotational depolarized scattering efficiency of about 1e-5 sr⁻¹, twoorders less than translational polarized for ‘P’ and four orders morethan translational polarized for ‘S’, the DC and AC signal strengths foreach of the four operating modes are estimated below. A signal-to-noiseratio of the order of unity or even lower is typically sufficient forreasonable correlation measurements.

Incidence- DC AC Signal-to- Measurement Detection Shutter Signal SignalNoise Ratio (1) Translation ‘P-P’ Closed 1e−8 1e−8 ~200 (2) AmplifiedTranslation ‘P-P’ Open 1e−6 1e−6 ~40000 (3) Rotation ‘P-S’ Closed 3e−93e−9 ~2 (4) Amplified Rotation ‘P-S’ Open 1e−6 2e−7 ~400

The significance of the above table remains true for a wide range ofdepolarization scattering efficiencies and mean relaxation rates. Itshould be noted that in modes (2) and (4) for a large homodyne gain, themeasured relaxation times are increased by a factor of two, which may beexploited by reducing the bandwidth accordingly.

Obviously, as the particle concentration, radius and ellipticity reduce,so does the depolarized signal. Smaller sizes also yield higherfrequencies and the additional noise further reduces signal-to-noiseratio, so that homodyne gain is proportionally more advantageous forsmaller particles, becoming almost essential in the 1 to 10 nm radiusrange. Homodyne gain is also almost essential for particles differingonly slightly from spherical, or of refractive index similar to thesuspending fluid.

Almost all of the observations, techniques, caveats, ideas andadvantages described in any of the 14 figures should also be consideredas applicable to any of the others where that may be possible or couldbe relevant.

The invention claimed is:
 1. An optical system for measuring particlecharacteristics comprising: a polarized illuminating beam of lightfocused in or near a specimen; polarizing components that selectivelypass either ‘S’ or ‘P’ polarized homodyne light to mix with at leastsome of the illuminating beam scattered from the specimen; polarizingcomponents that selectively pass either ‘S’ or ‘P’ polarized scatteredlight from the specimen; and a detector receiving both the polarizedhomodyne beam and polarized scattered light from the specimen, wherein aconjugate image of a field stop located adjacent the detector is formedat the specimen by a receiving lens and defines a first diameter of avisible volume of the specimen to be substantially equal to a seconddiameter of the polarized illuminating beam at the specimen.
 2. Theoptical system in claim 1 whereby the homodyne beam phase-front appearsto emanate from a region much smaller than the visible volume radius,while being conjugate with a common focal plane of illumination andfield stop images.
 3. The optical system in claim 1 whereby the opticalsystem comprises a shutter that blocks the homodyne light.
 4. Theoptical system in claim 1 whereby the optical system comprises apolarized homodyne beam transmitted through the specimen.
 5. An opticalsystem for measuring particle characteristics comprising: a polarizedilluminating beam of light focused in or near a specimen; polarizingcomponents that selectively pass either ‘S’ or ‘P’ polarized homodynelight to mix with at least some of the illuminating beam scattered fromthe specimen; polarizing components that selectively pass either ‘S’ or‘P’ polarized scattered light from the specimen; and a detectorreceiving both the polarized homodyne beam and polarized scattered lightfrom the specimen, whereby the optical system comprises an aperturepositioned to receive a visible cone of light from the specimen wherebythe aperture allows for more than one coherence area to fall upon adetector, and wherein a conjugate image of a field stop located adjacentthe detector is formed at the specimen by a receiving lens and defines afirst diameter of a visible volume of the specimen to be substantiallyequal to a second diameter of the polarized illuminating beam at thespecimen.
 6. An optical system for measuring particle characteristicscomprising: a light source; a sample region configured to hold a dropleton a single transparent plate; one or more optical components arrangedto focus an optical beam from the light source to an illumination volumehaving a first diameter at a location of the droplet on the transparentplate; one or more optical components arranged to receive scatteredlight from a visible volume at the location of the droplet and directthe received scattered light to a detector; and the detector positionedto receive the scattered light from the location of the droplet, whereina conjugate image of a field stop located adjacent the detector isformed at the location of the droplet by a receiving lens and defines afirst diameter of the visible volume of the droplet to be substantiallyequal to a second diameter of the illuminating beam at the location ofthe droplet.
 7. An optical system for measuring particle characteristicscomprising: a light source; a sample region configured to hold aspecimen within a translatable wedge shaped container; one or moreoptical components arranged to focus an optical beam from the lightsource to an illumination volume having a first diameter at the sampleregion; one or more optical components arranged to receive scatteredlight from a visible volume at the sample region and direct the receivedscattered light to a detector; and the detector positioned to receivethe scattered light from the visible volume at the wedge shapedcontainer, wherein a conjugate image of a field stop adjacent thedetector is formed at the specimen by a receiving lens and defines afirst diameter of a visible volume of the specimen to be substantiallyequal to a second diameter of the illuminating beam at the specimen. 8.An optical system for measuring particle characteristics by lightscattering, the optical system comprising: a light source; a sampleregion configured to hold a sample cell; a focusing lens arranged tofocus an optical beam from the light source to an illumination volumehaving a first diameter at the sample region; a receiving lens arrangedto receive scattered light from a visible volume at the sample regionand direct the received scattered light to a detector; and an aperturestop having a second diameter and located at the receiving lens, whereinthe second diameter admits more than one speckle, as defined by the vanCittert-Zernike coherence theorem, to the detector, wherein a conjugateimage of a field stop located adjacent the detector is formed at thespecimen by the receiving lens and defines a diameter of the visiblevolume at the sample region to be substantially equal to the firstdiameter of the illuminating volume at the sample region.
 9. The opticalsystem of claim 8, wherein the second diameter admits up to about 200speckles to the detector.
 10. The optical system of claim 8, wherein thesecond diameter admits up to about 1000 speckles to the detector. 11.The optical system of claim 8, wherein an intersection of theillumination volume and the visible volume is fully within the samplecell.
 12. The optical system of claim 8, wherein the scattered light isreceived in a rearward direction with respect to an incidence directionof the optical beam on the sample region.
 13. The optical system ofclaim 8, wherein the light source comprises a semiconductor laser. 14.The optical system of claim 8, further comprising: a beamsplitterarranged to split the optical beam from the light source and provide amonitoring beam; and a detection system configured to receive themonitoring beam and monitor one or both of an intensity and coherence ofthe optical beam.